Method of making a channel plate for a flat display device

ABSTRACT

A flat display device, preferably of the PALC type, in which the plasma channels are formed by etching in a substrate laterally-spaced channels and bonding a thin dielectric sheet over the etched substrate. Adjoining each of the channels are shallow ledges, also formed by etching, which serve as recessed areas to receive enlarged ends serving as contact pads for each of the electrodes. Holes are formed in the thin dielectric sheet and contact material deposited on the bonded thin dielectric sheet such that the deposited material makes electrical contact with the underlying electrode contact pads and seals off the holes, which allows a plasma-forming atmosphere to be provided in the channels. This arrangement results in a glass-to-glass interface between the substrate and the thin dielectric sheet, which allows anodic bonding to be employed to assemble the two elements and thus eliminates the frit glass sealing process required in other constructions.

This is a division of application Ser. No. 08/573,742, filed Dec. 18,1995 now U.S. Pat. No. 5,764,001.

RELATED APPLICATIONS

1) application, Ser. No. 08/384,090, filed Feb. 6, 1995 (PHA 60090).

2) application, Ser. No. 08/413,052, filed Mar. 29, 1995 (5604-0382) nowU.S. Pat. No. 5,596,431.

3) application, Ser. No. 08/573,742, filed Dec. 18, 1995 (5604-0394) nowU.S. Pat. No. 5,764,001

4) application, Ser. No. 08/588800, filed Jan. 19, 1996 (PHA 60099).

BACKGROUND OF INVENTION

This invention relates to plasma channels, to display devices comprisingplasma channels, and to plasma-addressed liquid crystal display panelscommonly referred to as "PALC" display devices using such channels. PALCdevices comprise, typically, a sandwich of: a first substrate havingdeposited on it parallel transparent column electrodes, commonlyreferred to as "ITO" columns or electrodes since indium-tin oxides aretypically used, on which is deposited a color filter layer; a secondsubstrate comprising parallel sealed plasma channels corresponding torows of the display crossing all of the ITO columns and each of which isfilled with a low pressure ionizable gas, such as helium, neon and/orargon, and containing spaced cathode and anode electrodes along thechannel for ionizing the gas to create a plasma, which channels areclosed off by a thin transparent dielectric sheet; and a liquid crystal(LC) material located between the substrates. The structure behaves likean active matrix liquid crystal display in which the thin filmtransistor switches at each pixel are replaced by a plasma channelacting as a row switch and capable of selectively addressing a row of LCpixel elements. In operation, successive lines of data signalsrepresenting an image to be displayed are sampled at column positionsand the sampled data voltages are respectively applied to the ITOcolumns. All but one of the row plasma channels are in the de-ionized ornon-conducting state. The plasma of the one ionized selected channel isconducting and, in effect, establishes a reference potential on theadjacent side of a row of pixels of the LC layer, causing each LC pixelto charge up to the applied column potential of the data signal. Theionized channel is turned off, isolating the LC pixel charge and storingthe data voltage for a frame period. When the next row of data appearson the ITO columns, only the succeeding plasma channel row is ionized tostore the data voltages in the succeeding row of LC pixels, and so on.As is well known, the attenuation of the backlight or incident light toeach LC pixel is a function of the stored voltage across the pixel. Amore detailed description is unnecessary because the construction,fabrication, and operation of such PALC devices have been described indetail in the following U.S. patents and publication, the contents ofwhich are hereby incorporated by reference: Buzak et al., "A 16-InchFull Color Plasma Addressed Liquid Crystal Display", Digest of Tech.Papers, 1993 SID Int. Symp., Soc. for Info. Displ. pp. 883-886.

A partial perspective view of the PALC display described in the 1993 SIDDigest is shown in FIG. 2. The method described in the referencedpublication for making the plasma channels is to chemically etch a flatglass substrate to form parallel semi-cylindrically shaped recessesdefined by spaced ridges or mesas and to bond on top of the mesas a thindielectric cover sheet having a thickness in the range of about 30-50μm. The bonding is typically carried out using a glass frit sealingprocess.

The above construction and its fabrication encounters certain problemsas a consequence of use of the glass frit sealing process. These involvethe presence of loose particles and contamination which detrimentallyaffects performance of the display.

SUMMARY OF INVENTION

An object of the invention is an improved channel plate.

A further object of the invention is an improved plasma-addresseddisplay device.

Another object of the invention is an improved method for fabricatingthe plasma channels of a PALC display device.

Still a further object of the invention is an improved method forfabricating the plasma channels of a PALC display device which avoidsthe use of a frit sealing process.

In accordance with a first aspect of the invention, a channel platecomprises a dielectric substrate and a thin dielectric sheet-like memberarranged over and spaced from the substrate by a plurality of laterallyspaced ridges or mesas formed in a major surface of the substrate anddefining a plurality of elongated spaced channels. At at least one endof each of the channels, at the same major surface, are formed shallowrecessed areas. The recessed areas serve to receive and locate theenlarged ends which serve as contact pads of spaced electrodes providedin each channel. Because the enlarged electrode ends lie below the majorsurface, glass frit sealing is no longer needed to bond the thindielectric sheet over the substrate to close off the channels.Preferably a direct glass-to-glass bonding process is employed, such asthe anodic bonding process described in application Ser. No. 08/384,090filed Feb. 6, 1995 (PHA 60090), whose contents are incorporated herein.

Preferably, the channels are formed in the substrate by an etchingprocess, and the recessed areas are formed adjacent to opposite channelends and along the longitudinal axis of each channel. Connections to thecontrol pads at the electrode ends are preferably made by way of viasformed in the thin dielectric sheet. The desired channel configurations,typically elongated parallel channels, are preferably straight but alsomay be curved while still maintaining a substantially parallelrelationship. The height of the mesas determines the height of thechannels, which are each formed by the portion of the substrate surfaceextending between adjacent flanking mesas, the flanking mesas themselvesforming the channel walls, and the overlying portion of the thindielectric sheet-like member. The assembled device incorporates aplasma-forming atmosphere.

It is known that glass can be anodically bonded to silicon using heatand an electric field to cause mobile ions in the contacting materialsto migrate to the sheet interfaces and bond them together.

In accordance with a second aspect of the invention, the contacts to thecontact pads are formed by deposition of metal on the thin dielectriccover sheet, with the result that the vias are sealed by the depositedmetal to close off the channels while at the same time completing thecontacts to the contact pads.

In accordance with a preferred embodiment of the invention, the channelplate is part of a PALC display device, and the combination of thesubstrate and the overlying thin dielectric sheet-like member, togetherwith the electrodes, constitutes the plasma channels or channel plate ofthe PALC display device.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its use,reference should be had to the accompanying drawings and descriptivematter in which there are illustrated and described the preferredembodiments of the invention, like reference numerals or letterssignifying the same or similar components.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic block diagram of a conventional flat panel displaysystem;

FIG. 2 is a perspective view of part of a conventional PALC displaydevice;

FIGS. 3A-7B illustrate the sequence of steps of one method in accordancewith the invention for fabricating one form of a channel plate accordingto the invention for use in a PALC color display. In this sequence offigures, in each instance, the A and B figures for each of the numberedfigures are partial top and crosssectional views along the line labelledB--B, respectively, showing the channel plate construction at each stepof the process.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a flat panel display system 10, which represents a typicalPALC display device and the operating electronic circuitry. Withreference to FIG. 1, the flat panel display system comprises a displaypanel 12 having a display surface 14 that contains a pattern formed by arectangular planar array of nominally identical data storage or displayelements 16 mutually spaced apart by predetermined distances in thevertical and horizontal directions. Each display element 16 in the arrayrepresents the overlapping portions of thin, narrow electrodes 18arranged in vertical columns and elongate, narrow channels 20 arrangedin horizontal rows. (The electrodes 18 are hereinafter referred to fromtime to time as "column electrodes"). The display elements 16 in each ofthe rows of channels 20 represent one line of data.

The widths of column electrodes 18 and channels 20 determine thedimensions of display elements 16, which are typically of rectangularshape. Column electrodes 18 are deposited on a major surface of a firstelectrically nonconductive, optically transparent substrate 34 (FIG. 2),and the channel rows are usually built into a second transparentsubstrate 36. Skilled persons will appreciate that certain systems, suchas a reflective display of either the direct view or projection type,would require that only one substrate be optically transparent.

Column electrodes 18 receive data drive signals of the analog voltagetype developed on parallel output conductors 22' by different ones ofoutput amplifiers 23 (FIG. 2) of a data driver or drive circuit 24, andchannels 20 receive data strobe signals of the voltage pulse typedeveloped on parallel output conductors 26' by different ones of outputamplifiers 21 (FIG. 2) of a data strobe or strobe means or strobecircuit 28. Each of the channels 20 includes a reference electrode 30(FIG. 2) to which a reference potential, such as ground, common to eachchannel 20 and data strobe 28 is applied.

To synthesize an image on the entire area of display surface 14, displaysystem 10 employs a scan control circuit 32 that coordinates thefunctions of data driver 24 and data strobe 28 so that all columns ofdisplay elements 16 of display panel 12 are addressed row by row in rowscan fashion as had been described. Display panel 12 may employelectro-optic materials of different types. For example, if it uses suchmaterial that changes the polarization state of incident light rays,display panel 12 is positioned between a pair of light polarizingfilters, which cooperate with display panel 12 to change the luminanceof light propagating through them. The use of a scattering liquidcrystal cell as the electro-optic material would not require the use ofpolarizing filters, however. All such materials or layers of materialswhich attenuate transmitted or reflected light in response to thevoltage across it are referred to herein as electro-optic materials. AsLC materials are presently the most common example, the detaileddescription will refer to LC materials but it will be understood thatthe invention is not limited thereto. A color filter (not shown) may bepositioned within display panel 12 to develop multi-colored images ofcontrollable color intensity. For a projection display, color can alsobe achieved by using three separate monochrome panels 12, each of whichcontrols one primary color.

FIG. 2 illustrates the PALC version of such a flat display panel usingLC material. Only 3 of the column electrodes 18 are shown. The rowelectrodes 20 are constituted by a plurality of parallel elongatedsealed channels underlying (in FIG. 2) a layer 42 of the LC material.Each of the channels 20 is filled with an ionizable gas 44, closed offwith a substantially transparent thin dielectric sheet 45 typically ofglass, and contains on an interior channel surface first and secondspaced elongated electrodes 30, 31 which extend the full length of eachchannel. The first electrode 30 is grounded and is commonly called theanode. The second electrode 31 is called the cathode, because to it willbe supplied relative to the anode electrode a negative strobe pulsesufficient to cause electrons to be emitted from the cathode 31 toionize the gas. As explained above, each channel 20, in turn, has itsgas ionized with a strobe pulse to form a plasma and a grounded lineconnection to a row of pixels in the LC layer 42 above. When the strobepulse terminates, and after deionization has occurred, the next channelis strobed and turned on. Since the column electrodes 18 each cross awhole column of pixels, only one plasma row connection at a time isallowed on to avoid crosstalk.

Fabrication of a PALC device is typically done as described in the 1993SID digest paper by providing first and second substrates 34, 36 withthe first substrate 34 comprising a glass panel on which isvapor-deposited the ITO column electrodes 18, followed by color filterprocessing over the ITO electrodes to produce the RGB stripes (notshown), followed by the black surround processing and liquid crystalalignment processing. The second substrate 36, also a glass panel, ismasked and etched to form the channels 20, following which the plasmaelectrode material is deposited and masked and etched to form thecathode 31 and anode 30 electrodes. A thin dielectric glass microsheet45 is then sealed across the peripheral edges of the device to form withthe ridges 50 the channels 20, which are then exhausted, back-filledwith a low-pressure ionizable gas such as helium and/or neon andoptionally with a small percentage of argon and sealed off. LC alignmentof the exposed surface of the microsheet 45 is then carried out. The twoassembled substrates are then assembled into a panel with the two LCalignment surfaces spaced apart and facing, the LC material 42introduced into the space, and electrical connections made to the columnelectrodes 18 and plasma electrodes 30, 31.

FIGS. 3A-7B are top and cross-sectional views of different steps in thefabrication of one form of channel plate in accordance with theinvention for one form of liquid crystal display panel in accordancewith the invention. A thick flat glass bottom plate 36 forms asubstantially transparent dielectric substrate for the plasma channels20. The first step is the etching of the elongated channels 20 from amajor surface 50 of the substrate. Only three channels are shown but itwill be appreciated that a typical PALC display device contains severalhundred channels. Also, the length of each channel (horizontal dimensionin the figures) typically equals the length of one row of the display.The channels 20 can be formed in any of several well known processes.Typically this is done by etching trenches in the substrate glass 36using standard masking and etching techniques, several of which aredescribed in the referenced patents and publications. For example, achromium masking layer on the substrate surface 50 can be used as theglass etchant mask, and the chromium mask can in turn be patterned usinga negative resist as a mask for the chromium etchant. The depth of theetched trenches 20 is substantially equal to the depth of the requiredplasma channels, for example 150 microns. After the channels 20 havebeen etched, the masking layer (not shown) is then patterned again toprovide at the end of each channel additional openings preferablyaligned with the longitudinal axis of the associated channel. Thesubstrate 36 is then etched further but only to a depth of a fewmicrons, to form shallow recessed areas or ledges 52 connected to andadjoining each channel end. The figures are not to scale. The depth ofthe ledges 52 need be only sufficient to accommodate the thickness of anenlarged contact end of each of the channel electrodes to be provided.The masking layer is then removed to leave the configuration illustratedin FIGS. 3A and 3B,

The electrode material can then be deposited and patterned in thestandard way, except that each electrode 54, 56 terminates,respectively, in an enlarged contact pad 58, 60 each of which is locatedalmost entirely in the ledge 52 at opposite channel ends. As will beappreciated by those skilled in this art, the cathode and anodeelectrodes, of which one is provided in each channel, typicallyterminate in contact pads at opposite channel plate edges, so that therequired potentials can be more easily applied to the electrodes in thecompleted device during operation. Thus all cathode electrodes 54 wouldtypically terminate, say, at the left edge, and all anode electrodes 56at the right edge of the plate. For clarity, the electrodes 54, 56 areshown stippled in the top views and hatched in the cross-sectionalviews. The plate at this stage is illustrated in FIGS. 4A and 4B.

The next step is to bond the thin dielectric sheet 45 over the substrate36 to close off the channels 20. Preferably, anodic bonding is used,which can be applied to any two bodies having surfaces containing ionsthat can become mobile at elevated temperatures. In a typical anodicbonding process, the thin dielectric sheet 45, typically of glass, isplaced over and in contact with the major surface 50 of the typicalglass substrate 36, the two are pressed together, and an electric fieldis applied across them while the assembly is heated to an elevatedtemperature. Ions migrate to the interface between the two sheets andpull them together. The resulting force, in the presence of the heat,leads to the formation of a permanent bond between the two sheets.Typical elevated temperatures are much lower than the softening point ofthe glass. The assembly at this stage is illustrated in FIGS. 5A and 5B.

After the bonding step, contact holes or vias 64 are then etched throughthe thin dielectric sheet 45, in the ledge area, over the contact pads58, 60, as illustrated in FIGS. 6A and 6B., preferably using vapor phaseetching, such as reactive ion etching.

Finally, as shown in FIGS. 7A and 7B, contact metal is then depositedover the top of the thin dielectric sheet 45 using conventionalevaporation or sputtering techniques. The contact metal chosenpreferably has good adhesion to glass, for example, Ti or Ti/W. Thecontact metal is then patterned using conventional masking and etchingtechniques to form the desired metal pattern to the outside world forultimately receiving the desired cathode and anode potentials. Thedeposited contact material, referenced 70, 72 for the correspondingcontact pads 58, 60, seals the contact holes 64 while making contact tothe underlying pads. The resultant glass plate 76 can then be pumped andfilled from the back of the substrate panel with the desiredplasma-forming atmosphere in the usual way by attaching a fill tube.

As an alternative procedure, it is possible to eliminate the fill tubeand filling and sealing procedure. This can be accomplished by carryingout the evaporation or sputtering of the contact metal to form theexternal contacts 70, 72 in the very same plasma-forming gas ambientwhich is to be used to fill the channels. For example, the sputtering orevaporation can be carried out in a gas atmosphere of, for example, Heor He/Ar, at the same pressure of the final plasma-forming atmosphere.Thus, at the conclusion of the metal deposition, which seals the vias64, the proper gas ambient will be sealed in situ in the channels.Therefore, the separate fill tube attachment, filling and sealingprocedures can be avoided.

The height of the channel sidewalls 78 produced during etching of thechannels, it will be appreciated, represents the height of the channels20 and constitute the spacers that space the thin dielectric sheet 45that closes off the channels from the substrate 36.

In addition to etching by conventional chemical etchants or byconventional plasma etching, alternatively, a mechanical erosion processcan be substituted, such as sandblasting.

As will be observed, because glass surfaces are directly in contact witheach other, or ion-containing layers on their facing surfaces, the twosheets 36, 45 can be anodically bonded to each other thereby avoidingthe frit sealing process. It will also be appreciated that theelectrodes 54, 56 must be deposited on the substrate 36 before the thindielectric sheet 45 has been bonded to it.

The resultant assembled channel plate structure 76 is shown in FIGS. 7Aand 7B. The remainder of the PALC panel can be fabricated in the usualway by forming the LC part of the panel on top of the thin glass sheet45 as shown in FIG. 2.

The term "deposited" as used herein means a layer formed by avapor-deposition process from a gas or vapor with or without an involvedchemical reaction, or by a sputtering or evaporation process.

The broken lines at the edges of the elements in the figures indicatethat what is shown is a small section broken off from a larger assembly,since, as will be appreciated, typically a PALC display device formonitor use would contain several hundred column electrodes 18 andseveral hundred plasma channels 20.

The materials for the electrodes 54, 56 are typically of a metal such ascopper, or layers of Cu--Cr--Cu, or other suitable metals.

All of the methods described in the referenced related applications andpublication, which are herewith incorporated by reference, will besuitable for making the remaining parts of the panel of the invention.

The invention is generally applicable to all kinds of flat displays, andin particular to displays of the plasma-addressed type, especially PALCdisplays that typically have a small channel pitch for use in computermonitors, workstations or TV applications. While the main application ofthe channel plate of the invention is in PALC type display devices, thesame plasma plate construction 76 can also be used as a plasma displaydevice where the output is the light, generated by the plasma, which canexit the device via the transparent substrate and/or the overlyingtransparent sheet-like member.

Several preferred examples for the FIG. 7 embodiment are (all values arein μm): a wall 78 width of about 20-50; a wall height of about 50-160;and a wall pitch of about 200-500.

It will be appreciated that the drawing figures are not to scale and inparticular the channel widths have been exaggerated to show theelectrodes.

Still further, while the channels in the substrate are typicallystraight, the invention is not limited to such a configuration and otherchannel shapes, such as a meandering shape, are also possible within thescope of the invention.

While the invention has been described in connection with preferredembodiments, it will be understood that modifications thereof within theprinciples outlined above will be evident to those skilled in the artand thus the invention is not limited to the preferred embodiments butis intended to encompass such modifications.

What is claimed is:
 1. In a method for making a channel plate for a flatdisplay device, said method comprising the steps of:(a) providing adielectric substrate, (b) providing a thin dielectric member, (c)forming in the substrate a plurality of substantially parallel channelsterminating in shallow ledges, (d) depositing an electrically conductivematerial over the substrate to form in each channel pairs of spacedelectrodes having ends, with the electrode ends extending into theshallow ledges, (e) bonding the thin dielectric member onto thesubstrate, (f) forming holes in the thin dielectric member at locationcorresponding to the locations of the electrode ends, and (g) forming onthe thin dielectric member deposited contact material that extendsthrough the holes in the thin dielectric member into contact with theelectrode ends in the shallow ledges.
 2. The method of claim 1, whereinthe substrate and the thin dielectric member, are constituted of glassand are bonded together by anodic bonding.
 3. The method of claim 1,wherein before step (d) is carried out, the holes are formed in the thindielectric member.
 4. The method of claim 1, wherein step (g) is carriedout so that the deposited contact material seals off the holes formed inthe thin dielectric member over the shallow ledges while simultaneouslymaking contact with the electrode ends extending into the shallowledges.
 5. In the method as claimed in claim 4, further comprising thestep of filling the channels with a plasma-forming atmosphere.
 6. Themethod of claim 4, wherein step (e) is carried out by anodically bondingthe thin dielectric member to the substrate.